The present invention relates to a rubber composition for a tire tread and a pneumatic tire comprising the rubber composition, more specifically a rubber composition for a tire tread, which is excellent in abrasion resistance, fuel efficiency and wet grip performance, and a pneumatic tire comprising the rubber composition.
Natural rubber has excellent raw rubber strength (green strength) compared to synthetic rubber and is excellent in processability. Also, natural rubber has high mechanical strength and excellent abrasion resistance as vulcanized rubber and therefore is frequently used in large tires such as truck/bus tires. However, because natural rubber only has methyl groups with a small molecular weight in the side chains and has a low glass transition temperature (Tg) of −50° C., grip performance is poor. For this reason, diene synthetic rubber is frequently used in automobile tires.
In recent years, concern is rising over rise in oil prices and exhaustion of oil due to problems of supply. Also, from the viewpoint of environmental problems such as tightening regulations for resource conservation and prevention of carbon dioxide exhaust, natural resources are being reconsidered. The tire industry is not an exception and as an alternative material of synthetic rubber, natural rubber is attracting attention again. In order to use natural rubber as an alternative material of synthetic rubber, improving fuel efficiency and grip performance without losing the excellent properties of natural rubber is an urgent task.
Conventionally, carbon black is used as a rubber reinforcing agent. However, in recent years, in view of social demands for energy and resource conservation, particularly to economize fuel consumption of automobiles, low heat generating properties in a rubber composition are also in demand. In order to decrease heat generating properties in a rubber composition, filling with a small amount of carbon black or using carbon black with a large particle size can be considered, but there is the problem that reinforcement properties and abrasion resistance decrease.
To solve this problem, the method of using silica as a filler to decrease heat generating properties in a rubber composition is known and up to the present, many patent applications have been filed regarding such a method (JP-A-3-252431). However, particles of silica tend to coagulate due to hydrogen bonds of silanol groups, which are surface functional groups. As a result, there are problems such as dipersability of the silica in rubber becomes low, the Mooney viscosity becomes high and processability, for example when extruding, is poor.
To solve this problem, a silane coupling agent is developed. The silane coupling agent bonds with the silanol groups and prevents coagulation of silica, improving processability. Also, in terms of performance, as a result of the silane coupling agent chemically bonding with silica and the polymer, rolling resistance is thought to decrease and abrasion resistance is thought to improve. In order to achieve such goals, silica and the silane coupling agent must be chemically reacted during kneading and thoroughly kneading at a high temperature is considered to be favorable to sufficiently react silica and the silane coupling agent. However, the functional group portion of the silane coupling agent, which reacts with rubber, reacts with some of the rubber due to the temperature during processing such as kneading and the phenomenon of scorching known as gelation occurs. When kneading is conducted at a low temperature at which scorching does not occur, the reaction of silica and the silane coupling agent is insufficient.
As another prior art, compounding a plasticizer, obtained by depolymerizing deproteinized natural rubber, to a rubber product in order to decrease the hysteresis loss of the rubber component is known (JP-A-9-249716). However, in a tire, improvement is desired in fuel efficiency and wet grip performance, without decreasing properties such as processability and mechanical strength.
Also, as a method for preparing modified natural rubber, which is modified with high efficiency, and providing modified natural rubber which does not cause allergies, known is the art relating to modified natural rubber, which is subjected to deproteinizing treatment and graft-copolymerization treatment with an organic compound having an unsaturated bond (Japanese Patent No. 3294903). However, an example of using such modified rubber as a rubber composition for a tire tread, with the purpose of improving fuel efficiency and wet grip performance, is not known.